Samples of rodent tumor or normal tissues from National Toxicology Program carcinogenicity studies, in in-house carcinogenicity studies, and from various other contract efforts continue to be collected and frozen for subsequent molecular biology studies. Approximately 12,000 samples have been frozen for use by various NIEHS investigators and over 1,200 of these samples have been or are in the process of being studied for oncogene and suppressor gene activities. All samples have been entered into a searchable data base and all tumors have been diagnosed and cataloged for orderly retrieval. Results from study of these samples is providing useful information for the interpretation of the biological significance of tumors in carcinogenicity bioassays, in understanding carcinogenesis, and in risk assessment for environmentally important chemicals. It is anticipated that this effort will continue over the next two years.